Over most of geologic time

In places like the Gulf of Mexico where land constrains the movement of the bulges, only one set of tides may occur each day. Inshore from an island there may be a complex daily cycle with four high tides. The island straits at Chalkis on Euboea experience strong currents which abruptly switch direction, generally four times per day but up to 12 times per day when the moon and the sun are 90 degrees apart.[34][35] Where there is a funnel-shaped bay or estuary, the tidal range can be magnified. The Bay of Fundy is the classic example of this and can experience spring tides of 15 m (49 ft). Although tides are regular and predictable, the height of high tides can be lowered by offshore winds and raised by onshore winds. The high pressure at the centre of an anticyclones pushes down on the water and is associated with abnormally low tides while low-pressure areas may cause extremely high tides.[32] A storm surge can occur when high winds pile water up against the coast in a shallow area and this, coupled with a low pressure system, can raise the surface of the sea at high tide dramatically. In 1900, Galveston, Texas experienced a 15 ft (5 m) surge during a hurricane that overwhelmed the city, killing over 3,500 people and destroying 3,636 homes.[36]

Sea level[edit source | editbeta]

Over most of geologic time, the sea level has been higher than it is today. Only at the Permian-Triassic boundary, around 250 million years ago, was the long-term sea level lower than it is now. The variations in sea level over time are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.[37] At the last glacial maximum, some 20 thousand years ago, the sea level was 120 metres (390 ft) below its present day level. For at least the last 100 years, sea level has been rising at an average rate of about 1.8 millimetres (0.071 in) per year.[38] Most of this rise can be attributed to an increase in the temperature of the sea and the resulting slight thermal expansion of the upper 500 metres (1,600 ft) of water. Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.[39] The rising trend is expected to continue

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